"Reports in the media have stated that WPP is investigating an allegation of financial impropriety by me, specifically as to the use of company funds," Sir Martin said in a statement.

"This allegation is being investigated by a law firm. I reject the allegation unreservedly but recognise that the company has to investigate it," he said.

"I understand that this process will be completed shortly."

Sir Martin added: "As a significant share owner, my commitment to the company, which I founded over thirty years ago, remains absolute - to our people, our clients, our shareholders and all of our many stakeholders. "